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Scanning Tunneling Microscopy
Wenbin Wang
Email:[email protected]
Course: Solid State II; Instructor: Elbio Dagotto; Semester: Spring 2009
Department of Physics Astronomy, University of Tennessee, Knoxville, TN 37996
April, 18, 2009
This paper begins with a brief introduction the development of scanning tunneling
microscopy (STM); in the second part, we will present some basic theory about STM,
we will pay more attention on the theory for tunneling between the surfaces of the
sample with the model probe tip. Next, we introduce the STM facility, include
different modes of operation and other influence factors. In addition, some application
of STM to nanofabrication and Nanocharacterization are reviewed. Finally,
conclusion and outlook are made towards the future research on STM.
Keywords: STM, Tunnel current, Nanofabrication
I.
Introduction
The 1986 years Nobel prize is a warded to Gerd Binnig and Heinrich Rohrer for their
design of the scanning tunneling microscope [1]. The invention of STM help scientist
better understand the surface structure which means this technical solved the
fundamental problem in surface physics. From 1986 until now, research go on
improve this advance technical and apply this method into industry innovations of
novel nanometer-scale product. Numerous advantages and versatility of STM make
itself become a powerful tool [3]. STM with tunneling spectroscopy can be used to
perform atomically resolved spectroscopy imaging functions such as energy-resolved
local density of states (LDOS) mapping, spin-polarized tunneling imaging, molecular
vibrational spectroscopy by inelastic tunneling, and tunneling-induced photon
emission mapping. Further more, as a nanofabrication tool, STM also be used in atom
manipulation, local deposition by the decomposition of organometallic gases.
Moreover, STM can be adapted to various environments such as air, ultrahigh vacuum
(UHV), controlled atmospheres and liquid [11]. In the following context, we will
introduce both basic theory of STM and application of STM in nano-device industry.
II.
Theory of STM
Tunneling current
From the figure 1(a,b,c), we will show some basic idea of tunnel [2]. In the figure
1( a), we can show the scheme of tunneling into vacuum, the electron in a valence
band are move in the Coulomb potential well of a atom core plus other valence
electrons, the wave function of the electron is extend into the vacuum. In the vacuum,
the wave function is decreasing exponentially, it tunnels into the vacuum. In real
materials, If two atoms come sufficiently close, just as shown in the figure 1( b), the
wave function of the two atoms overlapped with each other, at this time, an electron
can tunnel back and forth through the vacuum or potential barrier between them. In a
metal, in the figure 1 (c), the potential barriers between the atoms in the interior are
quenched, the electrons in the conduction
band can move
freely. as the same situation as a, at the
surface, the potential rises on the vacuum
side forming the tunnel barrier, only those
electrons which their electronic states very
near the Fermi level, can be excited and
tunnel across the barrier. As figure 1 (c)
shown, when the two metal surface are
closed enough, because the different
Fermi levels of those two metals which
produced by the small bias V, an electron
can tunnel to the surface atom of another
metal close by. The electron in the left
side of metal can tunneling to the right
side. There are many factors determine
this tunnel. First, tunneling require empty
level for the tunnel electron, so the
tunneling current is related with the
density of available states in the sample;
the current is also due to the bias voltage
V, when V is positive, electron in tip
tunnel into empty state in the sample
surface, if V is negative, electron tunnel
our of occupied states in the sample into the tip. The tunneling current is
 2
I    (r0 )  ( E  EF )

e
Ψν is the state of the surface; Eν is the energy of state Ψν in the absence of tunneling.
EF is the Fermi energy [6]. From this equation, we can found that the right part of this
equation is the surface local density of states (LDOS) at EF , so the tunneling current
is proportional to the surface LDOS. The microscope image represents a contour map
of constant surface LDOS. When a electron transmission an barrier, the tunneling
transitivity decreases exponentially with the distance of the barrier, in vacuum , when
the distance of the barrier decreases 1Å ,the current increases by an order of
magnitude, With one electrode shaped into a tip, the current flows practically only
from the front atoms of the tip, in the best case from a specific orbital of the apex
atom. This gives a tunnel-current filament width and thus a lateral resolution of
atomic dimensions. The second tip shown is recessed by about two atoms and carries
about a million times less current [4].
III.
Facility
A. Modes of operation
Basically, there are two kinds of modes for STM [5], one is called Constant current
which is the one we discussed in the following part and another one is named constant
height.
In the constant current mode, the tip is scanned across the surface at constant tunnel
current, vertical tip position will be continuously changed to keep the tunnel current
as a constant. Actually, in a homogeneous surface, constant current essentially means
constant interval between sample surface and the tip. In this method, the function
named height control mechanism will adjusting the tip move vertically up and down
to keep the tunnel current to be constant through the feedback voltage. That means in
this method, the tip topography the surface of the sample and gives a constant charge
density surface image .The feedback voltage is a
function of the height of the tip: Vz (Vx, Vy)
∝Z(x,y).When the surface of the sample is
smooth, there is another choice named Constant
height mode. In this mode, people through change
the current to make sure both the voltage and
height are held as a constant , the image of this
mode is made of current changes over the surface,
which can be related to charge density[8]. The
changing tunneling current as a function of
surface charge density conveys the same
information as obtained in constant current mode.
I(Vx, Vy) ∝√Ψ*Z(x,y). This mode is much faster
than the constant current mode. The only thing we
should pay attention to is the surface should be
smooth enough, or the scales of the ups and
downs may even larger than the distance between
the tip to the sample surface, at such situation, the
tip may be destroyed by crashing with the sample.
B. Instrument
To introduce the instrument, we use the constant tunnel current as a example, the
sketch of this instrument is shown in figure3 [7], the three frictionless x-y-z
piezodrive Px, Py, Pz, which is quite vibration sensitive, will control the movement of
the tip. Px, Py will scan the tip over the sample’s surface, Pz will depend on the
feedback voltage to determine the tip’s vertical move to keep the constant tunnel
current at constant tunnel voltage Vt. At the point B which is a contamination spot,
this lead the tip has a z direction displacement.
As we mentioned before, a little change on the
surface will lead a dramatic current change,
commonly, the barrier height or we called
work function Is nearly several electronvolts,
when we change the tunnel barrier width with
a single atomic step (2-5 Å), the tunnel
current will change about three orders of
magnitude. This is the basic principle for this
instrument. Include, in the constant tunnel
current modes, the surface topography has
constant work function and constant gap width.
C. Tip materials influence
There are lots of factors may influence the image of the STM, now, we introduce a
theory of STM imaging mechanism in terms of actual electronic states on real tip. If
we pay some attention to those researches about STM, we may found that the
commonly used tip materials are W, Pt and Ir [8], all of those materials are d-band
metals. Take tungsten tips as an example, we know that, at the Fermi level, 85% of its
density of states come from d states. Just as the some reference shown, the W atoms
on the tungsten tip surface will have a thinner electron density surrounded [9], the
reason for this phenomenon is because the tip atoms has experience a reduced force,
this force is come from the electron wind. In such situation, the w atom at the top of
the tip will show a positive charge, the electrical field will attract the W atoms move
to the apex of the tip, as a result, we can get very sharp tip with one atom at its apex,
this structure will gives a rise to the metallic d tip state. The early STM research think
that the tip materials have no relationship with the STM resolution, however, during
further research, we can see that the STM resolution is dependent on the tip materials
which require localized metallic Pz or dz [10] tip stats. From that research, we can
draw a conclusion that only d-bond metals and semiconductors which can form Pz
like metallic dangling bonds can be used as tip materials. All in all, the localized
surface states of the tip especially the chemical identity of the apex atom will help
people better understand the STM imaging mechanism.
IV.
Application
Just as mentioned before, because the advance characteristics and lots of advantages
of the scanning tunneling microscope (STM), STM become a powerful tool to
investigate surfaces of conductive materials since it’s invented. Recently, through the
trend of miniaturization both in magnetic and electronics devices, such as
high-density information storage, high-resolution lithography, and production of
nanoscale integrated chemical systems and electronic devices. So, now, the
development of advanced fabrication and characterization technologies and science
for the nanofunctional materials is become the most important problem in this
research area. As we know, the size of the advance micro devices is shrinking into the
nanometer scale, continuing miniaturization of electronic and mechanical devices has
led in recent years to an interest in the generation of nanometer-sized structures on
surfaces. To achieve this goal, we need to find an effective and efficient way to
resolve the bottleneck of nanotechnology. In this advance research area, the invention
of scanning probe techniques, particularly scanning tunneling microscopy (STM), has
made it possible to modify surfaces at the nanometer scale and to manipulate even
single atoms with the STM tip. STM
nanofabrication technique has potential
applications to various technologies. For
example, from the effect of single-electron
tunneling, a nanometer-scale SET
transistor is become an important
candidate technology for high-speed,
low-power large-scale integrated circuits
[11], which with tens of thousands of
transistors per chip. As figure 4 show,
technical for the fabricating nanometer
scale part such as nanowires and nanodots
on semiconductor substrate it become the
key point for the nanometerdevices.
A.
Z-piezo voltage pulse
As shown in figure 5, it is a STM image of a nanodot formed on a Si(111) surface
after applying a single z-pulse. The main techniques for this nanodevices is try to
fabricate metallic nanostructures on clean substrate. Now, let us show an example to
explain this process. The material of
the tip is made by tungsten wire with
thin silver film deposited on it. At the
beginning, we will apply a z-piezo
voltage pulse on the tip. The duration
time of this voltage pulse is 10sm. The
reason why we need to apply a voltage
pulse is because the adatoms on a
surface will always migrate toward the
center of the field which at that point,
the field is highest. As shown in the
figure 6 (a), in the absence of the
voltage pulse, surface potential is
periodic and the surface diffusion is in
random direction. When we apply a voltage pulse, at the position which near the tip,
the polarization energy is larger, this lead to the surface potential is inclined toward
the center. The atoms will be migrated toward the center, figure 6 (b) [15].
The whole process is shown in the figure, During the whole process, we use the
constant current mode. At the beginning, the tip is stay at a certain height, the gap
distance between the tip and the surface of Si(111) is approximately 0.5 nm, this gap
distance is changed with different materials and different tunneling conditions. Then,
we applied an externally generated voltage pulse directly to the z-piezo, which leads
to a corresponding movement of the STM tip in the z direction. When this additional
external voltage pulse is applied on the z-piezo, both tip height and the tunnel current
will show simultaneous responses. Just as shown in figure 7. In the first millisecond
of the pulse ( figure 8), the tunnel current show a sudden and dramatic increase,
showing that the STM tip approached the surface due to this additional voltage pulse
on the z-piezo [14]. Through the tip
is approaches to the surface, the gap
distance will be shorter, when the
gap distance is nearly to a typical
bond length or approaches the
atomic distance, the atoms on the
apex of the tip will transferred to the
sample surface and formed a
nanodot on the sample surface. After
3ms, the feedback circuit will return
a signal and causes the tip to retract
in order that the tunneling current
regains its predetermined value. All
this process seems very simple,
however, the principle of how the
atoms on the tip can be migrated
from the tip to the surface is
complex. There are two ways can
explain this transfer, one is called
Field evaporation model, the other is
caused by the spontaneous point contact.
Figure 8. Schematic model of tip-material
transfer mechanism by applying voltage pulse
C. Field evaporation model
We can get the general idea of this model
from the figure 9(a,b)
[15] and figure
10(a,b,c). When the distance between the tip
and sample surface are closed enough, a
potential barrier will formed, the atoms can
transfer between them. When we apply a field
between them, the energy barrier will be
lower. In a metal-vacuum-metal model, the
height of the barrier will be decreased a lot
when apply a field. In the process of
deposition, we apply a z direction voltage
pulse on the tip and sample surface, when the
tip and the surface are closed enough, the
barrier which the atoms will be tunneled is
very low. So the atoms can easier transfer
from the tip to the sample surface [16]. This is
the reason why atoms can be deposited on the
surface.
D. Spontaneous point contact
Another way to explain the atom transfer is use mechanical point-contact model. In
this model, tip-material transfer is caused by spontaneous formation of a point contact.
This can be confirmed by an abrupt jump in current, as shown in the figure 12. We
can prove this point contact from the I-z curve [13]. At the beginning, the tip is
gradually moving to the surface when we apply a z-direction voltage, the current is
exponentially increased through the tip close to the surface, the tip is in the tunneling
regime during this process. When the distance between tip and surface is close to a
typical bond length of -0.2nm, the current suddenly jumped to a very high value, this
dramatically change of current suggests the formation of a mechanical point contact
between the tip and Si(111) surface.
The reason for this atomic-sized
contact is various, it may be caused by
field-induced diffusion (high electric
field causes atoms at the tip Shank to
diffuse
to
the
apex
by
field-gradient-induced
surface
diffusion [15]) or by the formation of a
chemical bond.
Field-induced diffusion
The first model is called Field-induced diffusion. For this model, when we apply a
z-piezo voltage pulse on the tip, the field electrons can be emitted either from the tip
or the sample surface, this electron current will heat up or even melt the tip. For
another way, because there exist a field gradient on the tip surface, so the atoms on
the tip shank will flow to the tip apex either by a directional surface diffusion or by a
hydrodynamic flow. Then, at the apex of the tip , a liquidlike-metal cone will form,
the atoms on the apex of the tip are very easier to contact with the atoms on the
sample surface. After 10 ms, the voltage pulse is over and the liquidlike-metal cone
will cools down. The neck between the tip and the surface will be broken and a mount
of atoms migrated from the tip to the sample surface[15]. Just as shown in the figure
13.
Figure 13. Field-induced diffusion model of the atom contact
Chemical bond
Some other research shows that for some materials, we can use the chemical bond to
explain the mechanism for nanodots deposited. Such as Silicon, on the Si(111) surface
contains lots of adatoms, those adatoms have dangling bonds protruding into the
vacuum, when the gap distance is become very small, the dangling bonds will
extracting the tip, a nanoscale silver dot will left on the surface of the silicon because
the chemical bonding between the silver and silicon atoms [18]. After this, the
feedback circuit causes the tip to retract in order that the tunneling current regains its
predetermined value. This is the whole procedure for how tip-material transfer
mechanism from an tip to n-Si(111) by applying a z-direction pulse.
With the method we shown, people now can generate patterns on even a larger scale.
Figure 14 is a picture show an
20*20=400 single Cu clusters
array which is fabricated on an
area 245 nm by 275 nm , the
time of this fabricate is less
than 90 s [14]. The whole
process for this is the same as
we mentioned before. We were
able to achieve this relatively
fast patterning process by
apply the additional voltage
pulses on the z-piezo for the tip
approach at the right moments,
then the tip to approach the
surface while it was scanning
over the surface as in the usual imaging mode.
Figure 15 is a picture provided by IBM company which have positioned 48 iron atoms
into a circular ring in order to "corral" some
surface state electrons and force them into
"quantum" states of the circular structure [19].
The ripples in the ring of atoms are the
density distribution of a particular set of
quantum states of the corral. We were able
to achieve this relatively fast patterning
process by allowing the tip to approach the
surface while it was scanning over the surface
as in the usual imaging mode. It was
necessary to apply the additional voltage
pulses on the z-piezo for the tip approach at
the right moments.
V.
Conclusion
Basic theory and application of STM were reviewed. From the year 1986 Gerd Binnig
and Heinrich Rohrer receive the Nobel prize, STM has experienced a very fast
development during the past decades, nowadays, it already become the most
important method when research try to determine the surface structure. This method
first time provides a possibility of direct, real-space determination of surface structure
in three dimensions. Now, STM use its own advantages and combine its
nanofabrication and nanocharacterization capabilities together, become the most
powerful tool in the nanomaterial research field.
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